In mobile communication networks, there is always a challenge to obtain good performance and capacity for a given communications protocol, its parameters and the physical environment in which the mobile communication network is deployed.
Vectoring technology may be a core technology of the next generation of DSL (VDSL2; Very-high-speed digital subscriber line 2). It cancels the FEXT (far-end crosstalk) between DSL lines, and therefore maximizes DSL system performance. Vectoring technology will play a role in FTTX (Fiber To the Node/Cabinet Curb/Building/Home/Premises, etc.) business, because it enables offering 100 Mbps per user with DSL lines in the last hundred meters, i.e. between the end of a fiber network and the CPEs (Customer Premises Equipment).
ITU-T (Telecommunication Standardization Sector of the International Telecommunication Union) has been standardizing a vectoring standard G.993.5, and the first recommendation of G.993.5 was approved on April 2010. While G.993.5 specifies vectoring for VDSL2, the on-going next generation DSL standard ITU-T G.fast is now specifying vectoring as a mandatory feature. The cancellation of the FEXT is done at the DSLAM (Digital Subscriber Line Access Multiplexer side. The downstream FEXT is pre-cancelled by a precoder in the DSLAM, while the upstream FEXT is cancelled by an upstream crosstalk canceller in the DSLAM. The recommendation provides a way to estimate the FEXT channel in both downstream and upstream and utilize the estimated channel to cancel the crosstalk.
Recently, a reliability issue in vectoring referred to as CO (Central Office) disconnect event (CDE) has been observed. Particularly, some retrains have observed when a CDE happens. The retrain is caused by noise increase due to the CDE. This would interrupt the on-going services at least for the whole retrain time (up to 90 seconds). CDE occurs when one (or more lines) is disconnected unexpectedly at the CO side, e.g. at the MDF (main distribution frame). For example, this can be typical mistakes done by careless maintenance personnel. Such disconnect events may thus impair the performance of the DSL system.
Hence, there is still a need for an improved mitigation of disconnect events in vectoring based DSL systems.